This paper describes the design methodology adopted for stabilization measures of a twin bored highway tunnel in Northern Greece, where severe cracking and translational displacements of the temporary support shell were observed. Cracks occurred due to an active landslide in the vicinity of the tunnel that moves both bores downhill. A detailed numerical back-analysis based on actual geotechnical measurements data and crack 

observations on the support shell and the ground surface, was performed, in order to determine the possible sliding shear zone. Shear strength properties of the rockmass elements that failed in plastic yield were properly reduced, until the calculated support shell displacements reached the actual measured displacements. The effectiveness of different types of stabilization measures was investigated by using excessive multi-staged FDM (Finite Difference Method) analysis. The most effective combination of stabilization measures was selected in order to prevent additional ground movements and to stabilize the tunnel. Stabilization measures consist of reinforced concrete piles, pre-stressed permanent anchors and consolidation cement grouting applied on the periphery of the tunnels as well as on the intermediate pillar. The effect of potential seismic loading was numerically investigated.